Make <sys/systm.h> more self-contained.

Replace <sys/cdefs.h> with <sys/types.h>. Other includes need types
defined in <sys/types.h> and <sys/types.h> includes <sys/cdefs.h>
already.

While here, move the <machine/*.h> headers below the <sys/*.h>
headers.

Reviewed by: imp, kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D38841